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Monotreme
*** Shopping-Tip: Monotreme
{{Taxobox
| color = pink
| name = Monotremata
| image =Echidna_KI_.jpg
| image_width = 200px
| image_caption =
Short-beaked Echidna
| regnum =
Animalia
| phylum =
Chordate Chordata
| classis =
Mammalia
| ordo = '''Monotremata'''
| ordo_authority =
Charles Lucien Bonaparte C.L. Bonaparte, 1837
| subdivision_ranks = Families
| subdivision =
Kollikodontidae (''extinct'')
Ornithorhynchidae -
Platypus
Echidna Tachyglossidae -
Echidnas
Steropodontidae (''extinct'')
}}
'''Monotremes''' (''monos'', single + ''trema'', hole; refers to the
cloaca) are
Mammalia mammals that lay eggs, instead of giving birth to live young like
Marsupialia marsupials (
Metatheria) and
Placentalia placental mammals (
Eutheria). The subclass comprises a single order, '''Monotremata''' (though sometimes the subclass
Prototheria is used).
General characteristics
Like other mammals, monotremes are
homeothermic warm-blooded with high metabolic rates (though not as high as other
Mammalia mammals, see below); have
hair on their bodies; produce
milk to feed their young; have a single bone in their lower jaw; and have three middle ear bones (though this feature is now thought to have evolved independently in monotremes, see below).
Monotremes were very poorly understood for many years, and to this day some of the
19th century myths that grew up around them endure. It is still sometimes thought, for example, that the monotremes are "inferior" or quasi-
reptile reptilian, and that they are a distant ancestor of the "superior"
placentalia placental mammals. It now seems plain that modern monotremes are the survivors of an early branching of the mammal tree; a later branching is thought to have led to the
marsupial and
placental groups.
Similarly, it is still sometimes said that monotremes have less developed internal temperature control mechanisms than other mammals, but more recent research shows that monotremes maintain a constant body temperature in a wide variety of circumstances without difficulty, such as the
Platypus, which can maintain its body temperature even while living in an icy mountain stream. Early researchers were misled by two factors: Monotremes maintain a lower average temperature than most mammals (around 90°F / 32°C, compared to about 95°F / 35°C for marsupials, and 100°F / 38°C for most placentals); secondly, the
Short-beaked Echidna (which is much easier to study than the reclusive Platypus) only maintains normal temperature when it is active: during cold weather, it conserves energy by "switching off" its temperature regulation.
Image:Ornithorhynchidae-00.jpg thumb|left|[[Platypus]]
Physiology
The key physiological difference between monotremes and other mammals is the one that gave them their name; ''Monotreme'' means 'single opening' in Greek, and comes from the fact that their urinary, defecatory, and reproductive systems all open into a single duct, the
cloaca. This structure is very similar to the one found in reptiles. In contrast to the single cloaca of monotremes, other mammal females have separate openings for reproduction, urination and defecation: the vagina, the urethra, and the anus.
Monotremes lay
Egg (biology) eggs. However, the egg is retained for some time within the mother, who actively provides the egg with nutrients. Monotremes also lactate, but have no defined
nipples, excreting the milk via their skin. All species are long-lived, with low rates of reproduction and relatively prolonged parental care of infants.
Living monotremes lack teeth as adults. Fossil forms and modern platypus young have the "tribosphenic" (three-cusped)
molar (tooth) molars which are one of the hallmarks of mammals. However, recent work suggests that monotremes acquired this form of molar ''independently'' of placental mammals and marsupials (Luo et al, 2001). The jaw of monotremes is constructed somewhat differently from that of other mammals, and the jaw opening muscle is different. As in all true mammals, the tiny bones that conduct sound to the inner ear are fully incorporated into the skull, rather than lying in the jaw as in
cynodonts and other pre-mammalian
Synapsida synapsids; however this feature too is now claimed to have evolved independently in monotremes and
therians (Rich et al, 2005).
However, the external opening of the ear still lies at the base of the jaw. The monotremes also have extra bones in the shoulder girdle, including an interclavicle, which are not found in other mammals. Monotremes retain a reptile-like gait, with legs that are on the sides of rather than underneath the body. The monotreme leg bears a spur in the ankle region; the spur is non-functional in echidnas, but contains a powerful
venom (poison) venom in the male platypus.
Their metabolic rate is remarkably low by mammalian standards, although the extent to which this is a characteristic of monotremes, as opposed to an adaptation on the part of the small number of surviving species to harsh environmental conditions, is uncertain.
Taxonomy
The only surviving examples of monotremes are all indigenous to
Australia and
New Guinea, although there is evidence that they were once more widespread. Fossils of a jaw fragment 110 million years old were found at
Lightning Ridge,
New South Wales. These fragments, from species ''Steropodon galmani'', are the oldest known fossils of monotremes. Fossils from the genera ''Kollikodon'', ''Teinolophos'', and ''Obdurodon'' have also been discovered. In
1991, a fossil tooth of a 61-million-year-old platypus was found in southern
Argentina (since named ''Monotrematum'', though it is now considered to be an ''Obdurodon'' species). (See fossil monotremes below.)
* '''ORDER MONOTREMATA'''
** Family
Ornithorhynchidae: platypus
*** Genus ''
Ornithorhyncus''
****
Platypus, ''Ornithorhyncus anatinus''
** Family
Tachyglossidae: echidnas
*** Genus ''
Tachyglossus''
****
Short-beaked Echidna, ''Tachyglossus aculeatus''
*** Genus ''
Zaglossus''
****
Western Long-beaked Echidna, ''Zaglossus brujinii''
****
Sir David's Long-beaked Echidna, ''Zaglossus attenboroughi''
****
Eastern Long-beaked Echidna, ''Zaglossus bartoni''
Fossil monotremes
Excepting ''Ornithorhynchus anatinus'', all the animals listed in this section are extinct.
*Family
Kollikodontidae
**Genus ''
Kollikodon''
***Species ''
Kollikodon ritchiei''. Ancient monotreme 108-96 million years old.
*Family
Ornithorhynchidae
**Genus ''
Ornithorhynchus''. Oldest Ornithorhynchus specimen 4.5 million years old.
***Species ''
Ornithorhyncus anatinus'' (
Platypus). Oldest specimen 100,000 years old.
**Genus ''
Obdurodon''. Includes a number of
Miocene (5-24 million years ago) Platypuses.
***Species ''
Obdurodon dicksoni''
***Species ''
Obdurodon insignis''
***Species ''
Monotrematum sudamericanum''. 61 million years old.
(originally placed in separate genus, now thought an ''Obdurodon'')
*Family
Tachyglossidae
**Genus ''
Zaglossus''. Upper
Pleistocene (.1-1.8 million years ago).
***Species ''
Zaglossus hacketti''
***Species ''
Zaglossus robustus''
*Family
Steropodontidae. May be part of Ornithorhynchidae; closely related to modern platypus.
**Genus ''
Steropodon''
***Species ''
Steropodon galmani''.
**Genus ''
Teinolophos''
***Species ''
Teinolophos trusleri''. 123 million years old--oldest monotreme specimen.
References and external links
{{Wikispecies|Monotremata}}
{{Wikibookspar|Dichotomous Key|Monotremata}}
*Luo, Z-X, Cifelli, R. L., & Kielan-Jaworowska, Z. (2001). Dual origin of tribosphenic mammals. ''Nature, 409'', 53-57.
-
Rich, T. H., Hopson, J. A., Musser, A. M., Flannery, T. F., & Vickers-Rich, P. (2005). Independent origins of middle ear bones in monotremes and therians. ''Science, 307'', 910-914.
-
University of Tasmania's monotreme web site
-
UCMP Introduction to Monotremes
-
http://www.paleoportal.org/fossil_gallery/taxon.php?taxon_id=94
* {{MSW3 Groves|pages=p. 1-2}}
{{Mammals}}
Category:Monotremes
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